Patent GB-626133-A (GB)
Title: Improvements in or relating to radio direction finding
IPC: G01S1/02
Filed: 1946-07-15
Abstract: 626,133. Radio direction-finders. SADIRCARPENTIER. July 15, 1946, No. 21194. Convention date, Oct. 27, 1944. [Class 40 (vii)] [Also in Group XL (b)] A radio directionfinder comprises a pair of spaced non-directional aerials, the outputs from which are mixed with a locally' generated oscillation to provide signals having a predetermined intermediate frequency, one of which is periodically subjected to phase reversal and also to a continuous change of phase at a slower rate, after which the signals are heterodyned together, so that the amplitude of the heterodyne output is modulated due to the periodic phase reversal to an extent dependent upon the instantaneous value of the phase difference between the two signals. In the preferred embodiment, two pairs of aerials arranged at right angles are employed, the signals obtained from each pair being applied in alternation to the final indicator. The aerials are arranged in pairs A, B and C, D, Fig. 12, about a central aerial E which radiates the local oscillation which is combined with the received signals in mixers at the foot of each aerial. Considering the pair A, B, the output from the aerial A is subjected to a fixed phase-shift and applied, together with output from B which is subjected to a continuous cyclic variation of phase at a frequency f<1> by means of variable phaseshifting devices (see Group XL (b)), and is then periodically reversed in sign at a higher frequency f, to a mixer circuit, the output from which is then rectified. Thus, when the two signals applied to the mixer differ in phase by 90 degrees, the periodic phase reversal introduced does not affect the amplitude of the output of the mixer circuit and the rectified output is zero. At all other times there is obtained a rectified output of frequency f which is applied to one pair of deflecting plates of a cathode-ray tube, to the other pair of which a sweep voltage at the frequency f<1> is applied, so that a trace as shown in Fig. 8 is obtained. The distance OG is then an indication of the direction of reception. The display of signals received by the aerial pair A, B is alternated with the display of those received by pair C, D. The display is rotated through 90 degrees, Fig. 6 (not shown), when the pair C, D is used so that zero points are obtained on both the horizontal and vertical axes of the screen, since the distances of the zero points are proportional one to the cosine and the other to the sine of the direction of reception, the line joining these two points indicates the direction of reception. A transparent screen bearing a number of parallel lines, Fig. 10, may be mounted to rotate about the centre of the screen. These lines may then be set parallel to the line joining the zero points and the direction read off from a scale around the edge of the cathode-ray tube screen. The axes of the display may be graduated in degrees ; in this case, since the phase changes between the received signals are dependent upon their frequency, the amplitude of the sweep wave applied to the cathode-ray tube is made to vary in accordance with the frequency of the locally generated oscillation.